Display device and an electronic apparatus using the same

ABSTRACT

In a conventional display device comprising a sub-display, the display device is increased in thickness and in the number of components as the number of displays is increased. In the present invention, a dual emission display device is used so that either surface of a display is used as a main display or a sub-display. Accordingly, the display device can be reduced in thickness and in the number of components. Further, mechanical reliability can be enhanced when the invention is applied to a tablet PC, a video camera and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/969,232, filed Jan. 4, 2008, now allowed, which is a divisional ofU.S. application Ser. No. 10/799,756, filed Mar. 15, 2004, now U.S. Pat.No. 7,327,335, which claims the benefit of foreign priority applicationsfiled in Japan as Serial No. 2003-105923 on Apr. 9, 2003, and Serial No.2003-108484 on Apr. 11, 2003, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device having a displayscreen which is formed on a flat plate surface by using a light emittingelement typified by an electro luminescence element, and moreparticularly relates to an electronic apparatus using the displaydevice.

2. Description of Related Art

In recent years, with the development in communication technologies,portable telephone sets have been widely used. In future, transmissionof moving pictures and transmission of a large volume of information areexpected. On the other hand, through reduction in weight of personalcomputers (PCs), those adapted for mobile communication have beenproduced. Information terminals called PDA originated in electronicnotebooks have also been produced in large quantities and widely used.In addition, with the advance of display devices, most of those portableinformation apparatuses are equipped with a flat panel display.

Among active matrix display devices, manufacturing of a display deviceby the use of low temperature poly-silicon thin film transistors(referred to as TFTs hereinafter) is promoted. The use of lowtemperature poly-silicon TFTs has advantages in that in addition tomanufacturing of a pixel, a signal line driver circuit can be integrallyformed around a pixel portion in a display device. Thus, it is possibleto realize miniaturization and high definition of a display device, andsuch a display device is expected to be more widely used in future.

For portable PCs, tablet PCs have been developed. As shown in FIGS. 2Aand 2B, a tablet PC comprises a first housing 201, a second housing 202,a keyboard 203, a touch pad 204, a display portion 205 including a touchsensor, an axis of rotation 206, and a touch pen 207. When using thekeyboard 203 in such a tablet PC, as shown in FIG. 2A, data can be inputwith the keyboard 203 while looking at the display as in other notebookPCs. Meanwhile, when characters and data are input without using thekeyboard 203, as shown in FIG. 2B, the display portion 205 including atouch sensor and the touch pen 207 are used by rotating the firsthousing 201 in a complicated way and covering the keyboard 203 with thefirst housing 201.

As for portable telephone sets, bar-type phones have been replaced byflip phones, and among flip phones, those having two displays have beenactively developed. FIGS. 3A, 3B and 3C show an internal side, anexternal side and a lateral side of a flip phone, respectively. The flipphone shown in FIGS. 3A to 3C comprises a first housing 301, a secondhousing 302, a first display portion 303, a second display portion 304,a speaker 306, an antenna 307, a hinge 308, a keyboard 309, a microphone310, and a battery 311. As shown in FIGS. 3A to 3C, a sub-display (thesecond display portion 304) is provided as well as a main display (thefirst display portion 303), and thus the time, a battery charge status,a message reception status and the like can be displayed on thesub-display instead of on the main display (see Japanese Laid-OpenPatent Application No 2001-285445, for example).

SUMMARY OF THE INVENTION

In the above-described conventional tablet PC, the first housing isrequired to be rotated around the axis of rotation in a complicated wayin order to use one display portion in different positions. Therefore,mechanical reliability of the tablet PC is lowered as compared with a PCusing a simple hinge, thus machine life is shortened.

In the above-described portable telephone set comprising both a maindisplay and a sub-display, control circuits for controlling each of thedisplays are necessarily provided, leading to the increase in the numberof components, and the increase in volume and cost of the portabletelephone set. Further, the two displays make the first housing thickerand also increase the volume of the portable telephone set.

In view of the foregoing, it is a general object of the invention toprovide a display device having two displays while reducing the numberof components and volume and improving mechanical reliability. It isanother object of the invention to provide an electronic apparatus usingsuch a display device.

To solve the above-described problems, according to the invention, adual emission display is used, which serves as both a main display and asub-display. The dual emission display allows an electronic apparatus tohave a high reliability without using a complicated axis of rotation.

A display device according to the invention comprises a light emittingelement formed over a light transmissive substrate, wherein light fromthe light emitting element is emitted to the light transmissivesubstrate side and to the opposite side thereof so as to form a firstdisplay surface and a second display surface, and a first display screenformed on the first display surface is as large as a second displayscreen formed on the second display surface.

A display device according to the invention comprises a light emittingelement formed over a light transmissive substrate, wherein light fromthe light emitting element is emitted to the light transmissivesubstrate side and to the opposite side thereof so as to form a firstdisplay surface and a second display surface, and a first display screenformed on the first display surface is larger than a second displayscreen formed on the second display surface.

A display device according to the invention comprises a light emittingelement formed over a light transmissive substrate, wherein light fromthe light emitting element is emitted to the light transmissivesubstrate side and to the opposite side thereof so as to form a firstdisplay surface and a second display surface, and a plurality of displayscreens are formed on either the first display surface or the seconddisplay surface.

In the above-described display device according to the invention, thelight emitting element may emit white light and a color filter may beprovided on the side of the first display surface.

In the above-described display device according to the invention, thefirst display surface and the second display surface may be formed of aplurality of light emitting elements having different emission colors.

In the above-described display device according to the invention, thescan direction of the first display screen may be different from that ofthe second display screen.

In the above-described display device according to the invention, thefirst display screen and the second display screen may comprise a signalline driver circuit in common and the signal line driver circuit mayhave a switching means for changing the scan direction.

The above-described display device according to the invention maycomprise a volatile storage means and a switching means for changing thereading order of data stored in the volatile storage means.

In the above-described display device according to the invention, thefirst display surface and the second display surface may be sandwichedbetween at least two polarizers having different polarizationdirections.

The above-described display device according to the invention maycomprise a signal line driver circuit capable of arbitrarily selecting asignal line from a plurality of signal lines extending on the firstdisplay screen and the second display screen, and capable of outputtingan image signal to the signal line.

In the above-described display device according to the invention, aphotoelectric converter is provided on either or both of the firstdisplay screen and the second display screen.

An electronic apparatus according to the invention comprises a lightemitting element formed on a light transmissive substrate, a firsthousing and a second housing which are connected to each other so as tobe used both in open position and closed position, a display meansmounted in the first housing, which emits light from the light emittingelement to the light transmissive substrate side and the opposite sidethereof so as to form a first display surface and a second displaysurface, a detecting means for detecting a signal corresponding to theangle between the first housing and the second housing, and a switchingmeans for changing the scan direction of the display means in accordancewith a signal output from the detecting means.

An electronic apparatus according to the invention comprises a lightemitting element formed on a light transmissive substrate, and a displaymeans for emitting light from the light emitting element to the lighttransmissive substrate side and the opposite side thereof so as to forma first display surface and a second display surface, wherein a firstdisplay screen formed on the first display surface is as large as asecond display screen formed on the second display surface.

An electronic apparatus according to the invention comprises a lightemitting element formed on a light transmissive substrate, and a displaymeans for emitting light from the light emitting element to the lighttransmissive substrate side and the opposite side thereof so as to forma first display surface and a second display surface, wherein aplurality of display screens are formed on either the first displaysurface or the second display surface.

An electronic apparatus according to the invention includes a personalcomputer, a video camera, a digital camera, a portable communicationtool and the like each of which comprises a display screen.

An electronic apparatus according to the invention may comprise anelectrical storage means and a light emission control means for lightinga display screen when the electrical storage means is charged.

The above-described light emission control means may be added with afunction for lighting or flashing a display screen or an inverteddisplay screen whose contrast is inverted from that of a normal displayscreen, or a function for lighting a pixel which is less indeterioration. That is, this light emission control means may be addedwith a recording medium on which is recorded a control program forlighting or flashing a display screen, a recording medium on which isrecorded a control program for lighting or flashing an inverted displayscreen whose contrast is inverted from that of a normal display screen,or a recording medium on which is recorded a control program forlighting a pixel which is less in deterioration.

In a conventional portable telephone set having a sub-display, the twodisplays inhibit reduction in volume and cost of the portable telephoneset. Meanwhile, in a conventional tablet PC, the use of one displayrotated in a complicated way causes a low mechanical reliability.

According to the invention, a dual emission display functions as aplurality of displays and thus an electronic apparatus which is reducedin volume and cost and exhibits an improved mechanical reliability canbe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front elevational view of an embodiment mode of theinvention.

FIG. 1B is a rear elevational view of the embodiment mode of theinvention.

FIG. 1C is a side elevational view of the embodiment mode of theinvention.

FIG. 2A shows a conventional tablet PC in open position.

FIG. 2B shows the conventional tablet PC in closed position.

FIG. 3A shows an internal side of a conventional portable telephone set.

FIG. 3B shows an external side of the conventional portable telephoneset.

FIG. 3C shows a lateral side of the conventional portable telephone set.

FIG. 4A shows a tablet PC of the invention in open position.

FIG. 4B shows the tablet PC of the invention in closed position.

FIG. 5A shows an internal side of a portable telephone set of theinvention.

FIG. 5B shows an external side of the portable telephone set of theinvention.

FIG. 5C shows a lateral side of the portable telephone set of theinvention.

FIG. 6 is a circuit diagram of a source signal line driver circuit.

FIG. 7 is a circuit diagram of a gate signal line driver circuit with adecoder.

FIGS. 8A and 8B show a display panel with a built-in sensor.

FIG. 9 is a block diagram of a display controller.

FIGS. 10A and 10B show an embodiment of a dual emission display device.

FIGS. 11A and 11B show a video camera using the invention.

FIGS. 12A to 12D show a digital camera using the invention.

FIGS. 13A and 13B show a wristwatch communication tool using theinvention.

FIG. 14 is a block diagram of an electronic apparatus using theinvention.

FIG. 15A shows a pixel of an active matrix light emitting device.

FIG. 15B is a timing chart of the same.

FIG. 16A shows a pixel of an active matrix light emitting device.

FIG. 16B is a timing chart of the same.

FIGS. 17A to 17C show an embodiment mode of the invention in the case ofusing polarizers.

FIG. 18 is a schematic diagram of a dual emission display device.

FIG. 19 shows a pixel of a display device of the invention.

FIG. 20 is a block diagram of an electronic apparatus using theinvention.

FIG. 21 is a block diagram of a display controller.

FIG. 22 shows an electronic apparatus using the invention duringcharging.

FIG. 23 shows an example of a pixel circuit using the invention.

FIG. 24 shows a dual emission display panel using the invention.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

Although the present invention will be fully described by way of examplewith reference to the accompanying drawings, it is to be understood thatvarious changes and modifications will be apparent to those skilled inthe art. Therefore, unless otherwise such changes and modificationsdepart from the scope of the invention hereinafter defined, they shouldbe constructed as being included therein.

With reference to FIGS. 1A to 1C, Embodiment Mode of the invention willbe described hereinafter. FIG. 1A is a view of a display deviceaccording to the invention, seen from a first display surface, FIG. 1Bis a view of the same seen from a second display surface, and FIG. 1C isa side elevational view of the display device according to theinvention. In FIGS. 1A to 1C, the display device of the inventioncomprises light transmissive substrates 101 and 102, and display screens105 to 108. A display screen 105 is provided on the first displaysurface, and display screens 106 to 108 are provided on the seconddisplay surface. Driver circuits 103 and 104 for driving the displayscreens 105 to 108 are made up of TFTs and provided on the lighttransmissive substrate 101. An image signal and a control signal areinput to the driver circuits 103 and 104 through FPCs (Flexible PrintedCircuits) 109 and 110 each connected to the light transmissive substrate101, and thereby drive the display screens 105 to 108.

The display screen 105 on the first display surface displays an image byusing substantially the whole of the first display surface. Meanwhile,the display screens 106 to 108 on the second display surface occupy apart of the second display surface to display an image. Accordingly, thedisplay screen 105 can be used as a main display and the display screens106 to 108 can be used as sub-displays. In FIG. 1B, a received e-mail,the reception status of the radio wave, and the time are displayed onthe display screens 106, 107, and 108 respectively. However, displayedcontent is not limited to these and other content can be displayed.Further, the number of sub-display screens is not limited to three asshown in FIG. 1B, and one or more display screens can be providedarbitrarily.

As for a part of the second display surface, on which no image isdisplayed, a black display may be kept on or a black matrix may beprovided on this part, or the part may be covered with a material of thehousing when the display device is put in the housing.

It is to be noted that the number, the form and the size of sub-displayscreens are not limited to examples shown in FIG. 1B, and can bedetermined arbitrarily. The emission color of the main display screencan also be selected arbitrarily. For example, when a white lightemitting element is used for a light emitting element of the displaydevice, a full color display may be performed on the main display screenby using color filters, and a white display may be performed on thesub-display screens without using color filters. Alternatively, a lightemitting element including a plurality of emission colors may be used.

The display device according to the invention described above has abuilt-in driver circuit, though the driver circuit is not necessarilyimplemented in the display device. An LSI may be bonded with TAB or achip may be attached directly on a light transmissive substrate. Also,the display portion is not limited to an active matrix type usingpoly-silicon TFTs, and an active matrix type using amorphous TFTs or apassive matrix type may also be used.

FIG. 18 is a schematic diagram of a dual emission display deviceaccording to the invention. In FIG. 18, the display device compriseslight transmissive substrates 1801 and 1802, and transparent orquasi-transparent electrodes 1803 to 1805 and 1809 interposed betweenthe two substrates. Light emitting elements 1806 to 1808 are sandwichedbetween the electrodes 1803 to 1805 and 1809. On a surface of the lighttransmissive substrate 1801, color filters 1810 to 1812 are disposed.When the light emitting elements 1806 to 1808 emit a white light, a fullcolor display can be performed on the first light emitting surface and awhite display can be performed on the second light emitting surface. Thecolor filters are not necessarily provided and the light emittingelements 1806 to 1808 may include a plurality of emission colors. In thelatter case, the same color is displayed on each of the first lightemitting surface and the second light emitting surface. For the lightemitting elements 1806 to 1808, light emitting elements typified byelectro luminescence (EL) elements are used. According to such astructure, the dual emission display device can be achieved.

With reference to FIGS. 4A and 4B, an embodiment mode of the invention,which is applied to a tablet PC, will be described. The tablet PC shownin FIGS. 4A and 4B comprises a first housing 401, a second housing 402,hinges 409 and 410 for connecting the first housing 401 with the secondhousing 402, a keyboard 403, a touch pad 404, a main display screen 405,sub-display screens 406 to 408, and a touch pen 411. Since thesub-display screen 406 includes a touch sensor as described in therelated art, users can enjoy the advantage of the tablet PC by using thetouch pen 411.

In the conventional tablet PC described in the related art, the displaydevice comprises only one display portion and the housing is rotatedaround the axis of rotation in order to use the display portion for botha main display and a tablet display. In the embodiment mode shown inFIGS. 4A and 4B, however, the complicated axis of rotation described inthe related art is dispensed with by using the display device of theinvention shown in FIGS. 1A to 1C, and only the hinges 409 and 410 arerequired. According to such a structure, the mechanical reliabilitywhich is acknowledged as a problem in the conventional tablet PC can beenhanced.

FIGS. 5A to 5C shows an embodiment mode of a portable telephone setusing the display device of the invention. The portable telephone setshown in FIGS. 5A to 5C comprises a first housing 501, a second housing502, a first display screen 503, a second display screen 504, a thirddisplay screen 505, a speaker 506, an antenna 507, a hinge 508, akeyboard 509, a microphone 510, and a battery 511. FIGS. 5A, 5B, and 5Cshow an internal side, an external side, and a lateral side of theportable telephone set, respectively. The display device of theinvention is mounted in the first housing 501. Accordingly, the firsthousing 501 of the display device according to the invention can bereduced in thickness as compared with that of the conventional portabletelephone set shown in the related art, in which the two display devicesare mounted.

The number of sub-display screens is not limited to two as shown in FIG.5B, and one or three or more sub-display screens may be used.

Although the tablet PC and the portable telephone set are described inthis embodiment mode, the invention is not exclusively applied to these,and can be applied to electronic apparatuses using various displaydevices, such as a PDA, a video camera, a digital camera, a portable DVD(Digital Versatile Disc), a portable TV, and a game machine.

Embodiment 1

With reference to FIG. 6, explanation will be made on a source signalline driver circuit used for the display devices of the invention. Sinceimages are displayed on both sides of the display device in theinvention, the images are displayed in opposite directions viewing fromthe opposite side. Therefore, the driving direction of the screen has tobe changed depending on the direction from which the screen is viewed.Thus, in the display device of the invention, a source signal linedriver circuit is formed as shown in FIG. 6.

A source signal line driver circuit shown in FIG. 6 comprises a shiftregister 601, a NAND circuit 607, a buffer circuit 608, and analogswitches 609 to 612. The operation is explained hereinafter. The shiftregister 601 is formed with a series of DFFs 602, each of whichcomprises clocked inverters 603 and 604 and an inverter 605. A signal isinput to the DFF 602 from a terminal SSP and transferred to thesubsequent DFF 602 by clock signals (CL and CLK). A switch 606 iscontrolled by a SL/R to select whether to transfer a pulse to theprecedent stage or to the subsequent stage. When the precedent stage isselected by the switch 606, a pulse is transferred from left to right,and when the subsequent stage is selected, a pulse is transferred fromright to left.

These pulses are transferred to the NAND circuit 607 and then to thebuffer circuit 608 to drive the analog switches 609 to 612. Imagesignals are sampled through the analog switches 609 to 612 andtransferred to source signal lines S1 to S4.

In this manner, the switch 606 enables the image direction to change tothe left or right, and the source signal line driver circuit can beapplied to the dual emission display device of the invention. It is tobe noted that such driver circuit may be formed on a light transmissivesubstrate by using TFTs, an LSI may be bonded with TAB, or an LSI may beattached directly on a light transmissive substrate.

Embodiment 2

FIGS. 15A and 15B show an example of a light emitting display deviceusing time gray scale. FIG. 15A shows a pixel for driving a lightemitting element 1503 by using time gray scale. The pixel comprises adriving TFT 1502, a storage capacitor 1505 and a switching TFT 1501 aswell as the light emitting element 1503. A gate of the switching TFT1501 is connected to a gate signal line G1. When the gate signal line G1is high, the switching TFT 1501 is turned ON and data on a source signalline S1 is written to the storage capacitor 1505 and a gate of thedriving TFT 1502. When the driving TFT 1502 is turned ON, a current issupplied from a power supply line V1 to the light emitting element 1503through the driving TFT 1502. This state is held until the next writingis done.

FIG. 15B is a timing chart of time gray scale. In this embodiment, 4-bittime gray scale is taken as an example, though the invention is notexclusively limited to 4-bit. One frame is composed of four sub-framesSF1 to SF4. The sub-frames SF1 to SF4 each include address periods(writing periods) Ta1 to Ta4, and sustain periods (lighting periods) Ts1to Ts4, respectively. When the ratio between the sustain periodsTs1:Ts2:Ts3:Ts4 is set equal to 8:4:2:1, each bit corresponds to eachsustain period, thus time gray scale can be achieved. The addressperiods emit no light at this time, and perform only writing.

In order to drive the display device by using such time gray scale, adisplay controller and a memory are required for generating sub-frames.The display controller and the memory also enable to change the imagedirection to the left and right.

FIG. 9 shows a display controller and a memory. In this example, a 4-bitdigital video signal is divided into sub-frames, though the invention isnot exclusively limited to the 4-bit signal. The operation will bedescribed hereinafter. First, a display controller 902 inputs a digitalvideo signal to a memory A 904 through a switch 903. After all the datain the first, frame is input to the memory A 904, the switch 903 isswitched to a memory B 905 to write a digital video signal in the secondframe.

On the other hand, a switch 906 is sequentially connected to memories A904-1 to 904-4, and the signal stored in the memory A 904 is input to adisplay 901. After all the data in the second frame is input to thememory B 905, the switch 903 is switched to the memory A 904 to write adigital video signal in the third frame. Meanwhile, the switch 906 issequentially connected to memories B 905-1 to 905-4, and the signalstored in the memory B 905 is input to the display 901. By repeatingsuch an operation, sub-frames can be generated.

In the case where an image direction is changed to the left and right,signals are reversely called up per column of a display when the memoryA 904 or the memory B 905 is called up. In this manner, in the displaydevice in which sub-frames are generated, dual emission can be achievedby changing the order of calling up the memory.

Embodiment 3

FIGS. 16A and 16B show an example of a light emitting display deviceusing time gray scale. FIG. 16A shows a pixel for driving a lightemitting element 1603 by using time gray scale. The pixel comprises adriving TFT 1602, an erasing TFT 1606, a storage capacitor 1605 and aswitching TFT 1601 as well as the light emitting element 1603. A gate ofthe switching TFT 1601 is connected to a gate signal line G1. When thegate signal line G1 is high, the switching TFT 1601 is turned ON, anddata on a source signal line S1 is written to the storage capacitor 1605and a gate of the driving TFT 1602. When the driving TFT 1602 is turnedON, a current is supplied from a power supply line V1 to the lightemitting element 1603 through the driving TFT 1602. This state is helduntil the next writing is done.

FIG. 16B is a timing chart of time gray scale. In this embodiment, 4-bittime gray scale is taken as an example, though the invention is notexclusively limited to 4-bit. One frame is composed of four sub-framesSF1 to SF4. The sub-frames include address periods (writing periods) Ta1to Ta4, sustain periods (lighting periods) Ts1 to Ts4, and an eraseperiod Te. When the ratio between the sustain periods Ts1:Ts2:Ts3:Ts4 isset equal to 8:4:2:1, each bit corresponds to each of the sustainperiods, thus time gray scale can be achieved. By providing the eraseperiod Te in this pixel, it is possible to use time effectively. Lightcan be emitted in the address periods in the example shown in FIGS. 16Aand 16B, though it is not possible in the example shown in FIGS. 15A and15B. The erase period Te is required in the case where the lightingperiod is shorter than the address period. Thus, an erasing TFT 1606 andan erase line E1 are added for erasing in the pixel in FIG. 16A.

As in Embodiment 2, the image direction can be changed to the left andright by changing the order of calling up the memory circuit.

FIG. 19 shows an example of a pixel which is different from that in FIG.16A. FIG. 19 shows a pixel for driving a light emitting element 1903 byusing time gray scale. The pixel comprises a driving TFT 1907, anerasing TFT 1906, a storage capacitor 1905 and switching TFTs 1901 and1902 as well as the light emitting element 1903. A gate of the switchingTFT 1901 is connected to a gate signal line G1. When the gate signalline G1 is high, the TFT 1901 is turned ON, and data on a source signalline S1 is written to the storage capacitor 1905 and a gate of theswitching TFT 1902. When the switching TFT 1902 is turned ON, a currentis supplied from a power supply line V1 to the light emitting element1903 through the switching TFT 1902 and the driving TFT 1907. This stateis held until the next writing is done. A gate of the driving TFT 1907is connected to a power supply line V2 which has a fixed potential. Whenthe switching TFT 1902 is turned ON, a current corresponding to apotential difference between the V1 and the V2 is supplied to the lightemitting element 1903. Such a pixel is suitable for constant currentdrive in which the driving TFT 1907 is used in a saturation region.

Embodiment 4

FIG. 7 shows an example of a gate signal line driver circuit using adecoder. The decoder inputs address signals from address lines 1, 1 b,2, 2 b, 3, 3 b, 4, and 4 b to NAND circuits 701 and 702, and outputs theaddress signals to a gate signal line G001 through a NOR circuit 703 andinverters 704 and 705. In the above-described shift register, a signalline can not be selected arbitrarily because pulses are sequentiallyshifted, while in the decoder, a signal line can be selected arbitrarilyby performing addressing. Accordingly, by using the decoder, the displayscreen shown in Embodiment Mode can be lighted partially, and thelighted part can be used for a sub-display.

Embodiment 5

FIGS. 13A and 13B show a wristwatch communication tool using theinvention. FIG. 13A is the wristwatch communication tool in closedposition and FIG. 13B is the same in open position. Reference numeral1301 denotes a first housing and 1302 denotes a second housing. Thedisplay device of the invention is mounted in the second housing 1302.Reference numerals 1303 and 1304 denote belts, 1305 denotes a firstdisplay surface, 1306 denotes a second display surface, 1307 denotes acamera, 1308 denotes a keyboard, 1309 denotes a microphone, and 1310denotes a speaker.

In the case of using the communication tool in closed position, the timeis displayed on the first display surface 1305, and the communicationtool can be used as a common wristwatch. When using it in open position,various images can be displayed on the second display surface 1306. Whenthe communication tool has the videophone function, for example, it ispossible to display the face of the person at the other end of the line.Alternatively, the second display surface 1306 can be used as a Webterminal by accessing to the Internet. Also, other software applicationsor TV programs can be displayed on it. The first display surface 1305may display a message reception status or a battery charge status aswell as the time.

Embodiment 6

FIGS. 11A and 11B show a video camera using the invention. Generally ina video camera, a liquid crystal display is used for a monitor. When avideographer records a video image of others or objects, a monitor hasto be turned to the opposite direction of a camera lens. On the otherhand, when a videographer records a video image of himself, the monitorhas to be turned to the same direction of the camera lens. Therefore, itis necessary to rotate the monitor with respect to a main body of thecamera. Thus, a complicated axis of rotation as provided in theconventional tablet PC is required, which causes the decrease ofreliability. In the video camera using the invention, images can bedisplayed on both the first display surface and the second displaysurface. Accordingly, a simple hinge can be used as a substitute for acomplicated axis of rotation, thus reliability can be improved.

The video camera shown in FIGS. 11A and 11B comprises a main body 1101,a lens 1102, a microphone 1103, a finder 1104, a dual emission display1105, and a hinge 1106. The dual emission display 1105 includes a firstdisplay surface 1107 and a second display surface 1108. FIG. 11A showsthe case in which the dual emission display 1105 is used in closedposition and an image is displayed on the first display surface 1107.FIG. 11B shows the case in which the dual emission display 1105 is usedin open position and an image can be displayed on both the first displaysurface 1107 and the second display surface 1108. In this manner, amonitor can be used in either direction without a complicated axis ofrotation, leading to the improvement in the mechanical reliability.

Embodiment 7

FIGS. 12A to 12D show a digital camera using the invention. A digitalcamera in this embodiment comprises a main body 1201, a shutter 1202, afinder 1203, a lens 1204, a monitor display portion 1205, and a hinge1206. The monitor display portion 1205 includes a first display surface1207 and a second display surface 1208. A conventional digital camerahas a built-in monitor display portion which is fixed in it. In theinvention, the monitor display portion can be used in either openposition or closed position by using the dual emission display deviceand the hinge 1206. FIG. 12A is a front elevational view of the digitalcamera, FIG. 12B is a rear elevational view of the same, FIG. 12C is afront elevational view of the monitor in open position, and FIG. 12D isa rear elevational view of the monitor in open position. In this manner,an image can be monitored from either side with the monitor in openposition.

Embodiment 8

With reference to FIGS. 10A and 10B, an embodiment of the invention willbe explained. In this embodiment, a structure of a dual emission displaypanel including first and second display screens is described in detail.FIG. 10A shows an active element using transistors, and FIG. 10B shows apassive matrix element.

In FIG. 10A, a driving transistor 1001, a first electrode (pixelelectrode) 1002, a light emitting layer 1003, and a second electrode(counter electrode) 1004 are provided over a light transmissivesubstrate 1000. An overlapping area of the first electrode 1002, thelight emitting layer 1003 and the second electrode 1004 corresponds to alight emitting element 1025. In the invention, the first electrode 1002and the second electrode 1004 are formed of a light transmissivematerial. Therefore, the light emitting element 1025 emits light in thedirection of the substrate 1000 (first direction) and in the oppositedirection thereof (second direction), and has a first display region1005 and a second display region 1006. It is to be noted that for alight transmissive material of the first electrode 1002 and the secondelectrode 1004, used is a light transmissive conductive film such as ITOor aluminum having a thickness enough to transmit light.

In FIG. 10B, a first electrode (pixel electrode) 1060, a light emittinglayer 1061 and a second electrode (counter electrode) 1062 are formedover the light transmissive substrate 1000. An overlapping area of thefirst electrode 1060, the light emitting layer 1061 and the secondelectrode 1062 corresponds to the light emitting element 1025. Aninsulating layer 1063 and a resin layer 1064 are also provided to serveas banks.

As described above, the passive element has a structure in which thelight emitting layer 1061 is sandwiched between the electrodes. For thelight emitting layer 1061, a material composed mainly of inorganicsubstances may be used. In this case, an insulating layer may beprovided between the first electrode 1060 and the light emitting layer1061 or between the second electrode 1062 and the light emitting layer1061. For this insulating layer, aluminum oxide (Al₂O₃) and titaniumoxide (TiO₂) may be alternately laminated by thermal CVD usingabsorption reaction on a deposited surface.

This embodiment can be implemented in combination with otherembodiments.

Embodiment 9

With reference to FIGS. 8A and 8B, an embodiment of the invention willbe explained. In this embodiment, a structure of a dual emission displaypanel which includes first and second display screens and image sensoris described in detail.

In FIG. 8A, a driving transistor 801 and an emitting device 825 having afirst electrode (pixel electrode) 802 formed of a light transmissivematerial, a light emitting layer 803, and a second electrode (counterelectrode) 804 formed of a light transmissive material are formed over alight transmissive substrate 800. A light emitting element 825 emitslight in the direction of the substrate 800 (first direction) and in theopposite direction thereof (second direction). An insulating layer 835is formed on the second electrode 804. Over the insulating layer 835formed over the second electrode 804, provided are a photoelectricconverter 838 formed by laminating a P-type layer 831, an I-type(intrinsic) layer 832 and an N-type layer 833, an electrode 830connected to the P-type layer 831, and an electrode 834 connected to theN-type layer 833.

In the above mentioned dual emission display panel, the light emittingelement 825 is used as a light source, and the photoelectric converter838 is used as an image sensor. The light emitting element 825 and thephotoelectric converter 838 are formed over the same substrate 800. Alight emitted from the light emitting element 825 is reflected by anobject 837 and directed to the photoelectric converter 838. Then,potential difference between the two electrodes 830 and 834 of thephotoelectric converter 838 is changed and a current corresponding tothe changed potential difference is supplied between the two electrodes830 and 834. Thus, data of the object 837 can be obtained by detectingthe amount of the supplied current, and the obtained data can bedisplayed by using the light emitting element 825. It is to be notedthat when using an image sensor function, the object 837 is desirablyused closely contact with the display surface so that a light emittedfrom the light emitting element 825 as a light source is reflected bythe object 837.

In other words, the light emitting element 825 is used for bothdisplaying an image and for the light source in reading data of theobject 837. Further, the dual emission display panel has two functions:an image sensor function for reading data of the object 837 and adisplay function for displaying an image. In spite of having these twofunctions, it is not necessary to provide a light source and a lightdiffusing screen separately in the display panel, which are usuallyrequired for using an image sensor function. Therefore, the dualemission display panel of this embodiment allows the display device tobe reduced significantly in size, thickness, and weight.

With reference to FIG. 8B, explanation is made on an example of anequivalent circuit used for the above-described dual emission displaypanel. One pixel 850 is shown in FIG. 8B. The pixel 850 comprises asubpixel 817 including the light emitting element 825 and a subpixel 849including photoelectric converter 838. The subpixel 817 comprises asignal line 820, a power supply line 821, a scan line 822, a switchingtransistor 823 for controlling a video signal input, and a drivingtransistor 824 for supplying to the light emitting element 825 a currentcorresponding to the input video signal. It is to be noted that theconfiguration of the subpixel 817 can also be applied to a typicalcircuit configuration in a cross section of the transistor and the lightemitting element shown in FIG. 8A.

On the other hand, the subpixel 849 comprises a signal line 840, scanlines 842 and 843, a reset transistor 846 for resetting a potentialdifference between two electrodes of the photoelectric converter 838, anamplifier transistor 845 in which a current corresponding to thepotential difference between the two electrodes of the photoelectricconverter 838 is supplied between the source and drain thereof, and aswitching transistor 844 for controlling an input of a signal which isread from the photoelectric converter 838 to driving circuits.

It is to be noted that the active light emitting element and thephotoelectric converter are formed over the same substrate in thisembodiment, though the passive light emitting element and thephotoelectric converter as shown in FIG. 10B may be formed over the samesubstrate. Further, although each pixel comprises the light emittingelement 825 and the photoelectric converter 838 in this embodiment, thephotoelectric converter 838 is not necessarily provided in each pixel,and may be provided every several pixels in accordance with an object tobe read or usage of the portable terminal. Accordingly, the open arearatio of the light emitting element 825 is increased, thus bright imagescan be achieved.

This embodiment can be implemented in combination with otherembodiments.

Embodiment 10

FIG. 14 is a block diagram of a tablet PC using the invention. Thetablet PC in this embodiment corresponds to the one described inEmbodiment Mode. The tablet PC in this embodiment comprises a CPU 1401,a HDD 1414, a keyboard 1415, an external interface 1408, a nonvolatilememory 1407, volatile memory 1406, a communication circuit 1405, amicrophone 1412, a speaker 1413, an audio controller 1409, a touch panel1410, a touch panel controller 1411, a display controller 1404, a dualemission display 1403, and a display select circuit 1402. The dualemission display 1403 in this embodiment has to be changed the scandirection and images to be displayed depending on a used displaysurface.

In view of the foregoing, the invention provides a structure forselecting a display surface by detecting the angle of a hinge 1416 whichconnects a first housing with a second housing. When the hinge 1416 isin open position (when using the keyboard 1415), images corresponding toa main display are output. That is, the display select circuit 1402detects the angle data of the hinge 1416 and sends the data to the CPU1401. Then, the CPU 1401 instructs the display controller 1404 to senddata for the main display to the dual emission display 1403.

When the hinge 1416 is in closed position (when using a touch pen),images corresponding to a sub-display are output. That is, the displayselect circuit 1402 detects the angle data of the hinge 1416 and sendsthe data to the CPU 1401. Then, the CPU 1401 instructs the displaycontroller 1404 to send data for the sub-display to the dual emissiondisplay 1403. In such a manner, the display surface can be changed.

Embodiment 11

In this embodiment, another embodiment of a dual emission display panelused for the portable terminal of the invention is explained withreference to FIGS. 17A to 17C.

FIGS. 17A and 17B, reference numerals 1701 and 1702 denote polarizers,and 1703 denotes a dual emission display panel. FIG. 17A is a frontelevational view and FIG. 17B is a side elevational view. In thisembodiment, the dual emission display panel 1703 is interposed betweenthe polarizers 1701 and 1702. The two, polarizers are arranged so thattheir polarization directions cross each other, thus outside light canbe cut off. The crossing angle between the two polarization directionsis in the range of 40 to 90 degrees, preferably from 70 to 90 degrees,and if possible, at 90 degrees. Light from the dual emission displaypanel 1703 transmits through either of the two polarizers to display animage. Accordingly, a black display is performed in the region in whichno light is emitted and no image is displayed. Thus, the far side of thedual emission display panel is not transmitted in viewing from eitherside.

Either or both of the polarizers 1701 and 1702 may be rotatable, and thetransmittance of the dual emission display panel can be changed bychanging the crossing angle. That is, a brightness control function maybe additionally provided.

An antireflective coating or an antireflective film may be providedoutside the polarizers 1701 and 1702 in order to reduce the reflectance,and thereby improve the visual quality. Otherwise, a half-wave plate ora quarter-wave plate (or the relevant films) may be added. In thismanner, an added film having an optical function allows to enhance thevisual quality and in particular, to perform a black display moreclearly.

This embodiment can be implemented in combination with otherembodiments.

Embodiment 12

FIG. 22 shows a portable telephone set using the display device of theinvention, which is charging. In FIG. 22, a portable telephone set 2201is charged in open position by using a battery charger 2202, and displayportions emit light on both sides of the portable telephone set 2201.The portable telephone set 2201 may also be charged in closed position.Generally in a display device using a light emitting element, a lightemitting element deteriorates over time, and the brightness isdecreased. In the case of a display device having a light emittingelement in each pixel, in particular, lighting frequency of pixel isdifferent by location, thus degree of deterioration varies by location.Therefore, a higher lightning frequency causes a pixel to deterioratequickly and image persistence which decreases the image quality. Inorder to alleviate image persistence, an image is displayed duringcharging and the like, in which the portable telephone set is not usednormally, and a pixel less frequently used is lighted. As for an imagedisplayed during charging, used are an image displayed by lighting allthe pixels, an image obtained by inverting contrast of normal screen(idle screen, for example), an image displayed by detecting pixels lessfrequently used, and the like.

FIG. 20 is a block diagram corresponding to FIG. 22. In this embodiment,the portable telephone set 2201 comprises a CPU 2001, a HDD 2014, akeyboard 2015, an external interface 2008, a nonvolatile memory 2007, avolatile memory 2006, communication circuit 2005, a microphone 2012, aspeaker 2013, an audio controller 2009, a touch panel 2010, a touchpanel controller 2011, a display controller 2004, a dual emissiondisplay 2003, and a display select circuit 2002. A battery charger 2017detects a signal showing a charge status and sends the signal to the CPU2001. Then, the CPU 2001 instructs the display controller 2004 todisplay the corresponding signal, thus the dual emission display 2003emits light.

FIG. 21 shows an example of a controller for producing theabove-described image whose contrast is inverted from that of normaldisplay screen. An output signal from an image signal select switch 2106is input to a switch 2107. A display controller 2102 determines whetherthe output signal from the image signal select switch 2106 is inputdirectly to a display 2101 or input after being inverted. When contrasthas to be inverted, the signal may be input after being inverted. Aswitch 2103, a memory A 2104 and a memory B 2105 in FIG. 21 function asa switch 903, a memory A 904 and a memory B 905 in FIG. 9, respectively.In the case of displaying an image using all the pixels lighted, aconstant voltage may be input to the display 2101 (not shown).

In such a manner, deterioration of visual quality can be prevented byemitting light during charging so as to alleviate image persistence.This embodiment can be implemented in combination with otherembodiments.

Embodiment 13

Another example of pixel applied to the invention will be explained withreference to FIGS. 23 and 24. A pixel shown in a circuit diagram of FIG.23 comprises a light emitting element 2304, a switching transistor 2301for controlling a video signal input to the pixel, and transistors 2302and 2303 for controlling current supply to the light emitting element2304. In this embodiment, the transistor 2302 corresponds to a drivingtransistor, and the transistor 2303 corresponds to a current controltransistor. A capacitor 2305 for storing a video signal potential may beadditionally provided in the pixel as in Embodiment Mode of theinvention.

The driving transistor 2302 and the current control transistor 2303 havethe same conductivity. Both of them have P-type conductivity in thisembodiment, though they may have N-type conductivity. A thresholdvoltage of the driving transistor 2302 is set higher than that of thecurrent control transistor 2303, and more preferably, the drivingtransistor 2302 is to be a normally-ON transistor. Further, in theinvention, the ratio of the channel length to the channel width (L/W) ofthe driving transistor 2302 is larger than that of the current controltransistor 2303, thereby the driving transistor 2302 is operated insaturation region and the current control transistor 2302 is operated inlinear region. Specifically, in the driving transistor 2302, the channellength (L) is set longer, preferably five times longer or more, than thechannel width (W). Meanwhile, in the current control transistor 2303,the channel length (L) is set equal to or shorter than the channel width(W).

The gate of the switching transistor 2301 is connected to a scan line Gj(j=1 to y). One of the source and drain of the switching transistor 2301is connected to a signal line Si (i=1 to x) and the other is connectedto the gate of the driving transistor 2302 and the gate of the currentcontrol transistor 2303. The driving transistor 2302 and the currentcontrol transistor 2303 are connected in series. The driving transistor2302 and the current control transistor 2303 are connected to a powersupply line Vi (i=1 to x) and the light emitting element 2304 so that acurrent from the power supply line Vi is supplied to the light emittingelement 2304 as a drain current of the driving transistor 2302 and ofthe current control transistor 2303. In this embodiment, the source ofthe current control transistor 2303 is connected to the power supplyline Vi (i=1 to x), and the drain of the driving transistor 2302 isconnected to a pixel electrode of the light emitting element 2304.

The light emitting element 2304 comprises an anode, a cathode and anelectro luminescent layer interposed between the anode and the cathode.When the anode is connected to either the driving transistor 2302 or thecurrent control transistor 2303, the anode is used as a pixel electrodeand the cathode is a counter electrode. Meanwhile, when the cathode isconnected to either the driving transistor 2302 or the current controltransistor 2303, the cathode is used as a pixel electrode and the anodeis a counter electrode. A voltage is supplied from a power supply toeach of the counter electrode of the light emitting element 2304 and thepower supply line Vi so as to supply a forward bias current to the lightemitting element 2304.

One of the two electrodes of the capacitor 2305 is connected to thepower supply line Vi and the other is connected to the gates of thedriving transistor 2302 and the current control transistor 2303. Thecapacitor 2305 is provided to store the gate voltage of the drivingtransistor 2302 and of the current control transistor 2303, when theswitching transistor 2301 is turned OFF. Although the capacitor 2305 isprovided in FIG. 23, the invention is not exclusively limited to thisconfiguration, and the capacitor is not necessarily provided.

In the case where the source or the drain of the driving transistor 2302is connected to the anode of the light emitting element 2304, it isdesirable that the driving transistor 2302 has P-type conductivity. Onthe other hand, in the case where the source or the drain of the drivingtransistor 2302 is connected to the cathode of the light emittingelement 2304, it is desirable that the driving transistor 2302 hasN-type conductivity.

With reference to FIG. 24, explanation will be made on a structure of adual emission display panel which includes first and second displaysurfaces and uses the pixel circuit shown in FIG. 23. In FIG. 24, adriving transistor 2401, a current control transistor 2407, a firstelectrode (pixel electrode) 2402, a light emitting layer 2403, and asecond electrode (counter electrode) 2404 are provided over a lighttransmissive substrate 2400. The driving transistor 2401 is formed so asto be a normally-ON transistor. For example, elements such as boron,which imparts one conductivity, is added in a channel forming region byion implantation or ion doping.

A light emitting element 2425 is formed by laminating the firstelectrode 2402, the light emitting layer 2403 and the second electrode2404. In this embodiment, the first electrode 2402 and the secondelectrode 2404 are formed of a light transmissive material. Accordingly,the light emitting element 2425 emits light in the direction of thesubstrate 2400 (first direction) and the opposite direction thereof(second direction) and comprises a first display region 2405 and asecond display region 2406. It is to be noted that for a lighttransmissive material for the first electrode 2402 and the secondelectrode 2404, used are aluminum having a thickness enough to transmitlight, or a light transmissive conductive film such as indium tin oxide,zinc oxide, or indium tin oxide added with silicon oxide, gallium, zincoxide, tungsten oxide and the like.

This embodiment can be implemented in combination with otherembodiments.

1. A display device comprising: light emitting elements formed over alight transmissive substrate; a first display surface over one surfaceof the light transmissive substrate; and a second display surface overthe other surface of the light transmissive substrate, wherein an imageis displayed on a first display screen formed on the first displaysurface and a plurality of second display screens formed on the seconddisplay surface using a light from the light emitting elements.
 2. Thedisplay device according to claim 1, wherein the light emitting elementsemit white light, and a color filter is provided over the lighttransmissive substrate.
 3. The display device according to claim 1,wherein the light emitting elements emit different colored lights. 4.The display device according to claim 1, wherein a scan direction of thefirst display screen is different from a scan direction of the seconddisplay screen.
 5. The display device according to claim 4, wherein thefirst display screen and the second display screen comprise a signalline driver circuit in common, and the signal line driver circuitcomprises switching means for changing the scan direction of the firstdisplay screen and the scan direction of the second display screen. 6.The display device according to claim 4, wherein the display devicecomprises a volatile storage and a switching means for changing thereading order of data stored in the volatile storage.
 7. The displaydevice according to claim 1, wherein the first display surface and thesecond display surface are sandwiched by two polarizers having differentpolarization directions.
 8. The display device according to claim 7,wherein a crossing angle of the polarization directions of the twopolarizers is in a range of 45 to 90 degrees.
 9. The display deviceaccording to claim 1, wherein the display device comprises a signal linedriver circuit which is capable of arbitrarily selecting a signal linefrom a plurality of signal lines extending on the first display screenand the second display screen, and capable of outputting an image signalto the signal line.
 10. The display device according to claim 1, whereina photoelectric converter is provided on at least one of the firstdisplay screen and the second display screen.
 11. An electronicapparatus comprising: a light emitting element formed on a lighttransmissive substrate; and display means for emitting light from thelight emitting element to the light transmissive substrate side and theopposite side thereof so as to form a first display surface and a seconddisplay surface, wherein a first display screen is formed on the firstdisplay surface and a plurality of second display screens are formed onthe second display surface.
 12. The electronic apparatus according toclaim 11, wherein the electronic apparatus is a personal computer. 13.The electronic apparatus according to claim 11, wherein the electronicapparatus is a video camera.
 14. The electronic apparatus according toclaim 11, wherein the electronic apparatus is a digital camera.
 15. Theelectronic apparatus according to claim 11, wherein the electronicapparatus is a portable communication tool.
 16. The electronic apparatusaccording to claim 11, wherein the electronic apparatus comprises anelectrical storage, and light emission control means for lighting thefirst display screen and the plurality of second display screens whenthe electrical storage is charged.
 17. The electronic apparatusaccording to claim 16, wherein the light emission control meanscomprises a recording medium on which is recorded a control program forlighting the first display screen and the plurality of second displayscreens.
 18. The electronic apparatus according to claim 16, wherein thelight emission control means comprises a recording medium on which isrecorded a control program for lighting an inverted display screen whosecontrast is inverted from that of a normal display screen.
 19. Theelectronic apparatus according to claim 16, wherein the light emissioncontrol means comprises a recording medium on which is recorded acontrol program for lighting a pixel which is less in deterioration.